1. Field of the Invention
The present invention relates to a cutting method for cutting a workpiece with a laser beam, more particularly, though not exclusively, a laser beam condensed at an internal point of the workpiece.
2. Description of the Related Art
According to a conventional technique for cutting a workpiece (i.e., a member to be cut) with a laser beam, the workpiece such as a semiconductor substrate is cut by rotating, at a high speed, a circular blade having a width of several tens μm to several hundreds μm so that the abrasive material on the blade surface can grind the workpiece (semiconductor substrate). This method is generally referred to as a blade dicing method.
According to this method, to reduce heat generation or abrasion during the cutting operation, one can spray cooling water onto the cut surface. However, according to this method, finely powdered workpiece material or fine particles of the abrasive material generated during the cutting operation may be mixed with the cooling water and may spread in a wide region containing the cut surface.
To solve this problem, conventional systems perform the cutting operation in a dry environment without using cooling water. To this end, to cut a substrate, a processing method using a laser beam having a wavelength highly absorbable by the substrate can be employed to condense the laser beam on the substrate surface. However, according to this method, not only the portion directly irradiated with the laser beam but also its peripheral region on the substrate surface will melt. The electronic devices provided on the semiconductor substrate will be damaged.
On the other hand, there is a conventional processing method using a laser beam highly absorbable by a substrate to condense the laser beam inside the substrate so that the substrate can be cut by utilizing internal beam-condensing of the laser beam. For example, Japanese Patent Application Laid-open No. 2002-192370 and Japanese Patent Application Laid-open No. 2002-205180 discuss a processing method that uses a laser beam having a specific wavelength that shows high permeability relative to a substrate, i.e., a material to be cut, to condense the laser beam at an internal point of the substrate, thereby setting a start point of the cutting operation in a predetermined internal region of the substrate.
This processing method can realize an excellent cutting operation accompanied with less generation of heat or no hardening of dust particles on the surface, because no melt region is formed on the substrate surface.
Furthermore, Japanese Patent Application Laid-open No. 2002-205180 discusses a method for providing a plurality of property modified regions in the incident direction of a laser beam by changing the depth of the beam-condensing point.
However, according to the above-described conventional method, the start point of a cutting operation is limited to an internal region of the substrate where the laser beam is condensed. Hence, it can be difficult to appropriately control the direction or position of a crack growing from the start point of the cutting operation to the substrate surface.
Especially, in a workpiece made of a silicon wafer or a material having a similar crystal structure, the crack tends to grow along the crystal orientation. Therefore, according to the above-described laser processing method, if there is a small offset between a predetermined cutting line and a crystal orientation reaching the substrate surface that may be present due to manufacturing errors in the formation of the silicon substrate or elements, the crack may deviate from the predetermined cutting line in the process of growing toward the substrate surface and, as a result, may damage the logic circuits provided on a semiconductor element region.